WCOM125/COMP125
Fundamentals of Computer Science
Classes and Objects - 1
Gaurav Gupta
June 20, 2018
1
Contents
1 Introduction 4
2 Object Oriented Programming – Brief Introduction 5
3 Classes and instances 6
3.1 Example: Java’s String class . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 Example: String object . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3 Standard way of instantiatiation . . . . . . . . . . . . . . . . . . . . . . . 7
4 Class definition and instantiation 8
4.1 Defining classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2 Adding instance variables . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3 Defining methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.4 Example - Defining a class . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.5 Declaration and instantiation . . . . . . . . . . . . . . . . . . . . . . . . 10
4.6 Declaration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.7 Declaration and instantiation . . . . . . . . . . . . . . . . . . . . . . . . 10
4.8 Adding method to a class . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.9 The dot (.) operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.10 Are there any default values? . . . . . . . . . . . . . . . . . . . . . . . . . 13
5 Getters and setters 14
5.1 Bad client, bad bad client! . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2 Changing visibility to private . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.3 How does one access (read/write) private instance variables . . . . . . 15
5.4 Setters must provide validation where applicable . . . . . . . . . . . . . 15
5.5 I wish creating objects was easier . . . . . . . . . . . . . . . . . . . . . . 18
6 Constructors 19
6.1 Example - Constructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2 Constructors should call setters - always! . . . . . . . . . . . . . . . . . . 20
6.3 Default constructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.4 Defining the default constructor . . . . . . . . . . . . . . . . . . . . . . . 20
2
7 Displaying objects 22
7.1 Problem with the display() method . . . . . . . . . . . . . . . . . . . . . 22
7.2 Default toString() behaviour . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.3 Over-riding toString() behaviour . . . . . . . . . . . . . . . . . . . . . . 23
8 Class containing an array 25
9 Sample solutions for exercises 28
3
List of exercises
Exercise 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Exercise 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Exercise 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Exercise 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Exercise 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Exercise 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Exercise 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4
Section 1.
Introduction
Java is an object-oriented language. Classes and objects are one of the key program-
ming tools in Java. In this lecture we will cover some of the basic concepts about classes
and objects and how they are handled in Java.
5
Section 2.
Object Oriented Programming – Brief Introduction
• Classes are models of real world entities inside our programs.
• Classes may have instance variables and methods. For example, a Person class
can have a instance variable name of type String , and a method eat() of type
void .
• OO Design deals with designing the classes to solve a problem.
• OO Programming deals with realising that design correctly.
6
Section 3.
Classes and instances
Class definition specifies -
• instance variables each instance (object) of the class will have
• methods, including instance methods that are called on instances/ objects and
class methods or static methods that are called on the class directly without in-
voking objects.
ClassName
scope instanceVariable1 : type
scope instanceVariable2 : type
...
scope instanceMethod1 ( parameter : type, ... ) : return type
scope instanceMethod2 ( parameter : type, ... ) : return type
...
3.1 Example: Java’s String class
• As we know String is a built in class for handling sequences of characters.
• We can think of it as a Java type, just like int, double, boolean. (But String is a
non-primitive type.)
• Inside, it stores a sequence of characters (how, we don’t care right now).
• The interface provides a number of operations on the character sequence.
7
3.2 Example: String object
1 String greeting = "Hello, World!";
2 System.out.println(greeting.length());
Here, greeting is an instance, or, object of String class. The statement String greet-
ing = "Hello, World!"; is called the instantiation statement. The class has an instance
method length that is being invoked on object greeting
3.3 Standard way of instantiatiation
1 String lecturer1 = "Gaurav";
2 String lecturer2 = new String("Scott");
• The two variables are different instances of the String class.
• First is known as lazy instantiation, just for Strings.
• Second (using new) is generally how we create instances. More information about
this will come a bit later in this lecture ...
8
Section 4.
Class definition and instantiation
4.1 Defining classes
• Each class is defined in a separate file with the same name and ending with .java
• All Java class definitions are separate files in the same folder (for now).
4.2 Adding instance variables
Instance variables can be declared as in the following two examples. Note the public
modifier (for now):
1 public int instanceVar1;
2 public String instanceVar2;
4.3 Defining methods
Method definitions have a heading and a method body
• The heading defines the name of the method and other useful information.
• The body defines the code to be executed when the method is called.
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4.4 Example - Defining a class
1 public class Rectangle {
2 public double width;
3 public double height;
4 }
Defining a class: example Note that the above class definition merely provides a
template or blueprint for the class. No complete program using this class has yet been
written, and no object (instance) of this class has yet been created.
Rectangle
+ width : double
+ height : double
Exercise 1
Define a class
Define a class for a Circle that is represented by
its radius.
Write your answer here
(SOLUTION 1)
10
4.5 Declaration and instantiation
• An object or instance of the new class type is declared in main as follows:
1 ClassName classVar; //declaration
2 Rectangle r; //example
r
4.6 Declaration
Declaration creates a reference in the memory, which doesn’t refer to any storage space
yet.
4.7 Declaration and instantiation
• We perform the instantiation statement to allocate storage space for the in-
stance variables of the object declared and to refer to that memory.
1 cla s sVar = new ClassName ( ) ; // instantiation
2 r = new Rectangle ( ) ; // example
r
width = 0
height = 0
• These can be combined:
1 ClassName c l as s V a r = new ClassName ( ) ;
2 // declaration + instantiation
3
4 Rectangle r = new Rectangle ( ) ; // example
11
Exercise 2
Declare and Instantiate an object
Declare and instantiate an object myCircle of
class Circle
Write your answer here
(SOLUTION 2)
4.8 Adding method to a class
You can add methods inside the class that can be called on any instance of the class.
1 public class Rectangle {
2 public double width;
3 public double height;
4
5 public double area() {
6 return width
*
height;
7 }
8
9 public boolean isSquare() {
10 if(width == height)
11 return true;
12 else
13 return false;
14 //equally correct:
12
15 //return width==height;
16 }
17 }
Rectangle
+ width : double
+ height : double
+ area ( ) : double
+ isSquare( ) : boolean
4.9 The dot (.) operator
The dot operator gives us access to the members (instance variables and method) for
an object. Think of it as the apostrophe s (’s) of the human language (as in "Gaurav’s
class" or "Matt’s workshop")
1 Rectangle r = new Rectangle(); //example
2 r.width = 5;
• The expression r gives us access to the instance variable width of object r .
• When a method is invoked on an object using the dot operator, it calls the method
as defined in the class in the context of that object and any instance var iables
used in that method are the ones belonging to that object.
1 Rectangle r = new Rectangle();
2 r.width = 5;
3 r.height = 8;
4 System.out.println(r.area());
Here, r.area() returns width * height and since the method is called on object r , it
returns r.width * r.height . Had the method been called on another object s , it would
return s.width * s.height .
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Exercise 3
Access instance variables and call methods
Write a piece of code that sits outside the class
definition and displays the radius of the object
myCircle and also its area.
Write your answer here
(SOLUTION 3)
4.10 Are there any default values?
• Each instance variables is automatically initialised to the default value for its type
when an object of the class is created.
• For example, an instance variable of type int is given the default value 0;
• And an instance var iable of type String (or any class type) is given the default
value null. (More about null later.)
14
Section 5.
Getters and setters
5.1 Bad client, bad bad client!
Once the object is created, we can start operating on it.
1 Rectangle r = new Rectangle();
2 r.width = -5; // :o :O
3 r.height = 8;
4 System.out.println(r.area());
5 //displays -40 :'(
5.2 Changing visibility to private
1 public class Rectangle {
2 private double width;
3 private double height;
4
5 public double area() {
6 return width
*
height;
7 }
8
9 public boolean isSquare() {
10 return (width==height);
11 }
12 }
Rectangle
- width : double
- height : double
+ area ( ) : double
+ isSquare ( ) : boolean
15
Now, the instance variables width and height are visible only within the class
definition.
5.3 How does one access (read/write) private instance variables
We access (read and write) private instance variables through public methods called
getters and setters .
• getters return the value of the instance variable to the caller.
• setters set the value supplied by the caller to the instance variables.
1 public class Rectangle {
2 private double width, height;
3 public double getWidth() { return width; }
4 public void setWidth(double w) {
5 width = Math.abs(w);
6 }
7 //...same for height
8 //rest of the class definition...
9 }
5.4 Setters must provide validation where applicable
You can see that we validated the passed values before assigning to the instance vari-
able as width = Math.abs(w) . This is a typical case and setters are in charge of vali-
dating data before assigning it to the instance variables.
Rectangle
- width : double
- height : double
+ getWidth ( ) : double
+ getHeight ( ) : double
+ setWidth ( width : double ) : void
+ setHeight ( height : double ) : void
+ area ( ) : double
+ perimeter ( ) : double
+ isSquare ( ) : boolean
16
Exercise 5
Write a client
Write a client (code sitting outside Circle.java ,
for example, in the main method of another class)
that performs the following operations,
• Declare and instantiate object myCircle of
class Circle that has a radius of 1.8
• Display radius of myCircle .
• Increase radius of myCircle by 1.4
Write your answer here
(SOLUTION 5)
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5.5 I wish creating objects was easier
Let’s say that the user wants to create a Rectangle whose width is 5 and height is 8.
Following code achieves this,
1 Rectangle r = new Rectangle();
2 r.width = 5;
3 r.height = 8;
However, it would be really nice if one could pass the values for the instance vari-
ables in the instantiation statement itself, as,
1 Rectangle r = new Rectangle(5, 8);
This is done through constructors .
19
Section 6.
Constructors
• A constructor is a method defined in the class.
• A constructor must have the same name as the class.
• A constructor has no return type (not even void).
• There may be multiple constructors, each distinguished by its parameter list.
Thus, we may have one constructor with no parameters, and another with one
int parameter.
• A suitable constructor is automatically called during instantiation based on num-
ber of parameters passed. If an appropriate constructor is not found, a compi-
lation error is generated.
6.1 Example - Constructor
1 pu blic cl ass Rec tang le {
2 private d ouble width , h eight ;
3
4 // gette rs and setter s
5
6 publi c Rect angl e ( ) { // defaul t co nstructor
7 setWidth ( 1 ) ;
8 s etHeigh t ( 1 ) ;
9 }
10
11 // pa ra me te ri zed con st ru ct or for a square
12 publi c Rect angl e ( double s i d e ) {
13 setWidth ( s i d e ) ;
14 s etHeigh t ( side ) ;
15 }
16
17 // pa ra me te ri zed con st ru ct or - ge ne ric
18 publi c Rect angl e ( double w, do uble h ) {
19 setWidth (w) ;
20 s etHeigh t ( h) ;
21 }
22 // rest of the code
23 }
20
Rectangle
- width : double
- height : double
+ getWidth ( ) : double
+ getHeight ( ) : double
+ setWidth ( width : double ) : void
+ setHeight ( height : double ) : void
+ Rectangle ( )
+ Rectangle ( side : double )
+ Rectangle ( w : double, h : double )
+ area ( ) : double
+ perimeter ( ) : double
+ isSquare( ) : boolean
6.2 Constructors should call setters - always!
Constructors should always use setters to assign values to instance variables.
6.3 Default constructor
It should be noted that a default constructor (without any parameters) is pre-defined
for you by Java and that’s why you can instantiate objects without defining it yourself.
1 Rectange r = new Rectangle();
The default constructor assigns the default values for the appropriate data types to
the instance variables. However, once you define a parameterized constr uctor, the
built-in default constructor is taken away by Java. Thus, if you want to construct an
object with default initial values for the instance variables, you need to re-define that!
6.4 Defining the default constructor
Let’s say the default Rectangle instance should be of unit length. We can define the
default constructor as,
1 public Rectangle() {
2 setLength(1);
3 setBreadth(1);
4 }
21
Exercise 6
Add default and parameterized constructor
Add two constructors to the class Circle .
1. No parameters passed (default constructor):
Assigns the value 1.0 to radius ... through the
setter.
2. Parameter passed for radius (parameterized
constructor): Assigns the passed value to
radius through the setter.
Write your answer here
(SOLUTION 6)
22
Section 7.
Displaying objects
Often we need to display the details of an object. For example, we might need to dis-
play name and age of a Person object, or the details of a Time object in the format
hours:minutes:seconds , or in our example, width and height of a Rectangle ob-
ject. It is quite inconvenient to display these details as,
1 Rectangle r = new Rectangle(5, 8);
2 System.out.println(r.width+`` by ''+r.height);
We could add a method display in the class Rectangle as,
1 public void display() {
2 System.out.println(width+`` by ''+height);
3 }
And call this method on required object as,
1 Rectangle r = new Rectangle(5, 8);
2 r.display();
7.1 Problem with the display() method
But this would only let us display the object details, and not send to a file, or concate-
nate with any other output.
Java provides a standard way to return the String description of an object using the
toString() method (with return type String ).
7.2 Default toString() behaviour
When you display an object, what Java displays is the outcome of the toString() method
on that object
1 Rectangle r = new Rectangle(1, 3);
2 System.out.println(r); //something like [I@70dea4e
23
Java saw that you want to display a Rectangle object and replaced it by the toString()
method operating on that object as,
1 System.out.println(r);
2 //became
3 System.out.println(r.toString());
7.3 Over-riding toString() behaviour
We can over-ride toString() method as required. For the Rectangle class,
1 public String toString() {
2 return width+`` by ''+height;
3 }
When we display an object, it invokes the method toString() and displays the value
it returns.
1 Rectangle r = new Rectangle(5, 8);
2 System.out.println(r);
3 /
*
4 automatically invokes r.toString()
5 and displays the value returned
6
*
/
24
Exercise 7
Define toString method
Define the toString method in the Circle class
such that it displays the object details in the
following format -
Circle radius: <radius>, area: <area>
In a separate client, create a Circle object with
radius 1.6 and display it on the console.
Write your answer here
(SOLUTION 7)
25
Section 8.
Class containing an array
In this unit, we’ll extensively see classes containing arrays.
We must remember that both arrays and objects are references and refer to the
memory holding the actual data (in the case of array, it’s the array items, and in the
case of objects, it’s the instance variables).
Take the following as an example (instance variables are public for simplicity),
1 public class DynamicArray {
2 public int[] data;
3 public int nItems;
4 }
The client is as follows,
1 public class Client {
2 public static void main(String[] args) {
3 DynamicArray list = new DynamicArray();
4 }
5 }
At this stage, the memory state looks like,
26
If we change the values of the instance variables as,
1 public class Client {
2 public static void main(String[] args) {
3 DynamicArray list = new DynamicArray();
4 list.data = new int[4];
5 list.nItems = 0;
6 }
7 }
It now becomes,
list
data
nItems=0
[0]=0
[1]=0
[2]=0
[3]=0
Finally, we can modify the items of the array as,
1 public class Client {
2 public static void main(String[] args) {
3 DynamicArray list = new DynamicArray();
4 list.data = new int[4];
5 list.data[0] = 5;
6 list.data[1] = 12;
7 list.nItems = 2;
8 }
9 }
27
This gives us,
list
data
nItems=2
[0]=5
[1]=12
[2]=0
[3]=0
28
Section 9.
Sample solutions for exercises
Solution: Exercise 1
1 public class Circle {
2 public double radius;
3 /
*
4
*
note that int is a wrong choice as radius
5
*
CAN be a floating-point value like 1.5 or 2.4
6
*
/
7 }
Solution: Exercise 2
1 public class Client {
2 public static void main(String[] args) {
3 Circle myCircle = new Circle();
4 }
5 }
Although, you can just write the relevant part in written exams:
1 Circle myCircle = new Circle();
Solution: Exercise 3
1 System.out.println(myCircle.getRadius());
2 System.out.println(myCircle.area());
Solution: Exercise 4
1 //setter
2 public void setRadius(double r) {
29
3 if(r < 0)
4 radius = 0;
5 else
6 radius = r;
7 }
8
9 //getter
10 public double getRadius() {
11 return radius;
12 }
Solution: Exercise 5
1 public class Client {
2 public static void main(String[] args) {
3 Circle myCircle = new Circle();
4 myCircle.setRadius(1.8);
5 System.out.println(myCircle.getRadius());
6 myCircle.setRadius(myCircle.getRadius() + 1.4);
7 /
*
or you can split it up as:
8
*
double current = myCircle.getRadius();
9
*
double updated = current + 1.4;
10
*
myCircle.setRadius(updated);
11
*
/
12 }
13 }
Solution: Exercise 6
1 //default constructor
2 public Circle() {
3 setRadius(1);
4 }
5
6 //parameterized constructor
7 public Circle(double r) {
8 setRadius(r); //let setter handle the validation
30
